The following description sets forth the inventor's knowledge of related art and problems therein and should not be construed as an admission of knowledge in the prior art.
As an extrusion die for manufacturing a metal hollow extruded product, such as, e.g., an aluminum heat exchanging tube for use in a heat exchanger for car air-conditioners, there are a porthole die as shown in FIG. 16A, a spider die as shown in FIG. 16B, and a bridge die as shown in FIG. 16C.
In these extrusion dies, a male die 1 and a female die 2 are combined with the mandrel 1a of the male die 1 placed in the corresponding die hole 2a of the female die 2 to define a circular extrusion hole by and between the mandrel 1a and the die hole 2a. A metal billet (metallic material) pressed against the billet pressure receiving surface of the male die 1 is introduced into both the dies 1 and 2 via material introduction holes 1c formed in the male die 1 and then passed through the extrusion hole while being plastically deformed, so that an extruded member having a cross-section corresponding to the cross-sectional configuration of the extrusion hole is formed.
In such an extrusion die, since a large stress due to the pressing of the metal billet is applied to the billet pressure receiving surface 1b of the male die 1, the stress may cause a generation of cracks in the billet pressure receiving surface and therearound, which in turn may result in insufficient die life.
Under the circumstances, an extrusion die for metallic material as disclosed by the below-listed Patent Documents 1 and 2 is conventionally proposed. This is a bridge die with the bridge portion of the male die fitted to the female die. In this die, the billet pressure receiving surface of the male die is formed into a convex configuration which projects in a direction opposite to the extrusion direction of the billet to avoid adverse effects due to the pressing of the metal billet by receiving the pressing force of the metal billet in a pressing force reduced manner with the convex surface.    Patent Document 1: Japanese Unexamined Laid-open Utility Model Publication No. S53-102938 (see claims, and FIGS. 3 to 5)    Patent Document 2: Japanese Unexamined Laid-open Patent Document No. H02-280912 (see claims and FIGS. 1 to 3)
In the conventional extrusion die shown in the aforementioned Patent Documents 1 and 2, since the billet pressure receiving surface is formed into a convex configuration, the bridge portion is insufficient in strength, although the strength of the male die, such as the resistance to pressure against a metal billet, is improved to some extent. Therefore, in order to secure sufficient strength of the bridge portion, the size of the male die such as the thickness of the bridge portion has to be increased beyond necessity, which results in not only an increased size and weight but also an increased cost.
Especially in the case of extruding an extruded article having a complicated configuration using an extrusion die, it is necessary to stably and smoothly introduce the metal material into the extrusion hole from the material introducing portion. In the aforementioned conventional extrusion die, however, the metallic material which flows from the material introducing portion of the male die into the space between the male die and the female die is disturbed by the bridge portion of the male die. This prevents smooth introduction of the metallic material, causing deteriorated dimensional accuracy of the extruded article, which in turn makes it difficult to attain high quality.
The description herein of advantages and disadvantages of various features, embodiments, methods, and apparatus disclosed in other publications is in no way intended to limit the present invention. Indeed, certain features of the invention may be capable of overcoming certain disadvantages, while still retaining some or all of the features, embodiments, methods, and apparatus disclosed therein.
Other objects and advantages of the present invention will be apparent from the following preferred embodiments.